A photosensor using a CCD has a great advantage in that only one amplifier is needed for the output signal because it has a simple one output terminal. But its rather narrow dynamic range has limited the application. For example, a common spectrophotometer requires a very wide dynamic range which the conventional normal CCD photosensor cannot cover.
Japanese laid-open patent publication No. S62-76765 discloses one solution to the problem: the floating diffusion of the output section of a CCD device is given a knee-shape characteristic to prevent saturation or overflow. This widens the dynamic range, but the output from this device does not have a linear relationship with the strength of the sensed light, and needs a linearlization circuit. Further it is difficult to obtain a stable knee-characteristic due to manufacturing deviations. Japanese laid-open patent publication No. S63-63928 proposes another solution: the charge-generating time (integration time) is switched from one to the other. This device also requires an additional device control, and it cannot respond to a momentary change in the light strength.